To better understand implantation of the blastocyst in the endometrium in non-human primates, these studies focus on cellular interactions of the trophoblast and endometrium, and on differentiation of the early postimplantation embryo and trophoblast. Five macaque implantation stages, between 2-3 days and 6-7 days after onset of implantation, were used in immunohistochemical studies. Immunostaining of adjacent sections was achieved by serial sectioning of implantation sites. Multiple antibodies, particularly the combination of antibodies to cytokeratin and to neural cell adhesion molecule (NCAM), helped determine the pathway of the cytotrophoblast into the endometrial stroma and to identify endovascular cytotrophoblast. Antibody to vimentin, Factor VIII (endothelial cells) and desmin (smooth muscle) showed components of endometrial vessels, and indicated that the preferential pathway for invasion of the endometrium is intravascular, and only secondarily interstitial. Cytotrophoblast invasion into arterioles, which become partially occluded within two days after onset of implantation, is remarkably rapid. However, only individual trophoblast cells enter the superficial venules. Since endovascular trophoblast in both venules and arterioles displays the NCAM adhesion molecule, this is not a differential molecule for intravascular invasion; it remains unknown why cytotrophoblast does not advance into dilated venules beyond the level of the fetal-maternal junction. Invasion of the endometrium during formation of the trophoblastic shell occurs at a rate that could not be sustained without maternal damage. These studies, and others showing the potential of cytotrophoblast cells to become invasive, indicate that maternal regulation of interstitial cytotrophoblast invasion is a significant part of normal placentation. This species would be useful in determining how trophoblast cells penetrate into arterioles and what factors determine the extent of their invasion. Since the remodelling of maternal vessels by endovascular cytotrophoblast is important in development of normal placental circulation, factors regulating these features of placentation are important areas of study. *KEY* placenta, Trophoblast, Endometrium, Immunocytochemistry, Electron microscopy